1. Field of the Invention
This invention relates to a foldable propeller and more particularly a boat propeller of the type comprising a propeller hub and propeller blades movable carried by the hub in such a manner that they may be folded from a working position in which they project radially outwards from the propeller hub to a rest position in which they are directed backwards in line with the propeller shaft.
2. Prior Art
Several versions of propellers of this type, which are generally used with auxiliary engines for sailing boats, are already known. In the simplest version, the propeller blades unfold under the influence of the centrifugal force, both when forward propulsion is applied, and when propulsion in the reverse direction is applied. In some cases the inner ends of the propeller blades are interconnected in such a way that the blades will move simultaneously.
It has also been suggested that the blades should be designed in such a way that they do not cover each other completely in folded position. With such a design, parts of the edges of the blades will "grip" the water at the moment of starting, so that the blades will unfold because of the pitch of the blades.
Furthermore, a propeller design has been suggested where the propeller hub comprises elements which can be mutually turned to a limited extent, one element being immovably attached to the propeller shaft while the other element carries the foldable propeller blades. An axially movable member is coupled to the blades and, when the hub elements are turned in relation to each other, this member is axially moved to unfold the blades. The idea is, thus, that the propeller blades will be forced to unfold both when forward propulsion is applied and when propulsion in the reverse direction is applied.
The constructions described above have in common that they function comparatively well during forward propulsion. Irrespective of whether the main unfolding influence is the centrifugal force or the circumstance that the blades are also shaped in such a way that they "grip" the water and unfold because of the pitch of the blades, or whether the control device described above is used to achieve the unfolding, reasonably reliable function is achieved only during forward propulsion. When propulsion in the reverse direction is applied, however, satisfactory and reliable function cannot be achieved, especially not if a quick and sudden change of propulsion direction is required while the boat is moving forward at any noticeable speed, since the propeller blades must then unfold, and must stay unfolded, against the thrust exerted by the water current.